When a DC voltage is supplied into one terminal (e.g., an input terminal), it may be desired to switch states between the one terminal and another terminal (e.g., an output terminal) from an electrically-conductive state to a cut-off state, based on a voltage level of the DC voltage. In such applications, a protection switch circuit is used. FIG. 1 shows a circuit diagram of a protection switch circuit that the present inventors have studied.
A protection switch circuit 100r has an input (IN) terminal and an output (OUT) terminal. If an input voltage VIN input to the IN terminal falls within a predetermined voltage range, a state between the IN terminal and the OUT terminal of the protection switch circuit 100r becomes conductive. The protection switch circuit 100r includes a switch 102, a gate controller 104, an overvoltage protection (OVP) circuit 110a, and an undervoltage lockout circuit 110b. 
The switch 102 is provided between the IN terminal and the OUT terminal. The overvoltage protection circuit 110a compares the input voltage VIN with a threshold VOVP for overvoltage protection. The threshold VOVP corresponds to an upper limit of a voltage range. The undervoltage lockout circuit 110b compares the input voltage VIN with a threshold VUVLO for undervoltage lockout. The threshold VUVLO corresponds to a lower limit of the voltage range. The gate controller 104 turns on the switch 102 if VUVLO<VIN<VOVP or turns off the switch 102 if VIN<VUVLO or VOVP<VIN, according to outputs from the overvoltage protection circuit 110a and the undervoltage lockout circuit 110b. 
In the protection switch circuit 100r of FIG. 1, it may be desired to adjust the thresholds VUVLO and VOVP. In a system where a power supply voltage, in addition to the input voltage VIN, is constantly supplied to the protection switch circuit 100r, the thresholds VUVLO and VOVP may be adjusted, since the protection switch circuit 100r includes built-in registers to store setting data indicative of the thresholds VUVLO and VOVP so that the setting data may be written to the built-in registers by an external processor.
However, in another system where an input voltage VIN is used as a power supply voltage of protection switch circuits 100r, it is not possible to read from or write to registers if the input voltage VIN is not supplied to the protection switch circuits 100r. In such a system, an adjusting method of the thresholds VUVLO and VOVP by hardware processing is adopted. A fuse-trimming by laser cutting, a zapping by fusing the aluminum wiring by an electric current or the like are examples of hardware processing.
Such hardware processing causes the manufacturing cost of a protection switch circuit 100r to increase. In addition, since the hardware processing may be performed only during the manufacturing process of the protection switch circuit 100r, a user of the protection switch circuits 100r, in other words, a manufacturer of a set apparatus equipped with the protection switch circuit 100r may not adjust the thresholds.